This invention is in the field of microcalorimetry. This invention relates generally to devices for performing calorimetry at small scales. In another aspect, the invention relates to methods of making calorimetric measurements.
A calorimeter is a widely used analytical instrument that measures heat release or absorption by a sample under test. The main components in calorimeters are heating and temperature sensing elements such that the sample under test can be heated and cooled in a temperature controlled manner. A differential scanning calorimeter (DSC) is one of the most routinely used calorimeters for characterizing metals or polymers, in which fast scanning in wide temperature ranges is possible while rejecting common-mode signals, thus enhancing sensitivity. Since DSC is a differential metrology, the reference and sample must be thermally decoupled. While both heaters are operated with a fixed temperature ramp, the differential power is measured and provides information regarding phase transition, temperature corresponding to and heat accompanied by a specific transition, such as a glass transition, crystallization, melting, or decomposition.
Microelectromechanical systems (MEMS) technologies have aided the development of microfabricated calorimeters, such as a DSC. Because of its small size, a microDSC consumes orders of magnitude smaller sample volumes, as well as offering fast operation and improved sensitivity. Since a microDSC can analyze sample quantities ranging from microgram to nanogram, for example, it offers the possibility to analyze costly or rare material samples, and can also be used to measure size-dependent thermodynamic properties.
Several different configurations have been offered for a MEMS-based DSC. In general, most microfabricated calorimeters previously reported have employed a dielectric structural membrane and a metal heater and thermometer. The thermal time constant of the previous designs are often limited by the low thermal conductivity of the dielectric membrane. Moreover, the metal heater and thermometer generally have long-term stability issues caused by electromigration. For example, a thin-film DSC (TDSC) consisting of a metal heater and thermometer on a thin dielectric film has been fabricated and used to measure size-dependent depression of melting point and reduction of enthalpy of fusion of thin metallic films and glass transition of thin polymeric films.
A suspended bridge small-scale DSC having a heater and temperature sensor isolated from and connected to a bulk chip via tethers was reported to achieve better temperature uniformity in the heater and a rectangular micro hotplate type microDSC has been employed for combustible gas sensing. Most previous works on micro differential scanning calorimetry focused on reducing thermal capacity of the device but in general there is a lack of detailed thermal design analysis and experimental validation regarding heating efficiency and response time.
International Application Publication No. WO 2006/073426 discloses a microcalorimeter comprising a semiconductor substrate having a thin membrane of a low heat capacity material (e.g., Si3N4 or SiC) deposited on a suspended plate and includes a gold or chromium heater and a thin film AuGe thermometer.
U.S. Patent Application Publication No. US 2004/0195096 discloses a calorimetric type gas sensor comprising a suspended porous silicon membrane having an insulating layer of SiO2 and/or Si3N4. A doped polycrystal silicon or Pt/Ti conducting heater is formed on top of the insulating layer. A second insulating layer is formed thereon, followed by a catalytic (e.g., Pt or Pd) layer deposited on top of the second insulating layer.
International Application Publication No. WO 2007/026177 discloses a gas sensing semiconductor device fabricated from a silicon substrate including an insulating SiO2 layer, doped single crystal silicon resistive heater, and source and drain CMOS circuitry. An inorganic (e.g., SnO2) or organic (polymer or pthalocyanine) gas sensing region is separated from the heater by a second insulating layer.
U.S. Pat. No. 5,451,371 discloses a gas sensor comprising a pair of polysilicon plates each having a Pt heating resistor and Pt temperature sensing resistor. A silicon nitride passivation layer covers the Pt resistors and a catalytic layer (e.g., Pt, Pd) is deposited on one of the polysilicon plates.
U.S. Pat. No. 6,436,346 discloses a micro calorimetric sensor formed from silicon or silicon dioxide having a platform with a resistive Platinum thermal detector thereon. A chemical coating is formed on the platform for operation of the sensor, and temperature control of the platform is achieved by adding a polysilicon heating resistor to the platform or a Peltier stage to the entire sensor.